The present invention relates generally to medical ventilators and, more specifically, to tubing for use in that portion of the ventilator commonly known as the breathing circuit, which includes the gas supply and control paths and the patient's respiratory system.
The breathing circuit includes an inhalation conduit for delivering gas to the patient, an exhalation conduit for receiving gas from the patient, a patient wye at which the inhalation and exhalation conduits join, and an endotracheal tube or tracheostomy outlet for interfacing the ventilator and patient sides of the breathing circuit. Another conduit may connect the patient wye to the endotracheal tube or tracheostomy outlet. The breathing circuit may also include filters located at the junctions between the exhalation and inhalation conduits and the ventilator housing, a humidifier, and other devices.
Tubular fittings on the patient wye, filters and other portions of the breathing circuit receive the conduits. The conduits include a length of flexible tubing and may also include endpieces for connecting the tubing to the fittings. One commonly used type of fitting is a barbed, cylindrical or frusto-conical projection. The end of the flexible tubing is stretched over this fitting. Alternatively, the end of the flexible tubing may be attached to an elastomeric endpiece, which is, in turn, stretched over the fitting. Another commonly used type of fitting, which is defined by the ISO 5356-1 standard, consists of a rigid frusto-conical female half, into which a male half attached to the end of the flexible tubing is inserted. The male and female halves are formed of a rigid material such as metal. Nevertheless, the friction between the mating halves maintains the connection.
The conduits are an extremely critical aspect of the breathing circuit. The coupling between a conduit and the fitting to which it is connected must withstand tensile stresses to prevent disconnection. The tubing must withstand flexure resulting from normal movement of the patient or ventilator without kinking or collapsing and thus restricting gas flow. Failure of the tubing or coupling can cause damage or death to a patient. In fact, an industry standard attachment test requires that a conduit remain connected to a fitting when a 20 pound tensile force is exerted for 60 seconds. The tubing is typically ribbed, such as by providing corrugation or a continuous spiral rib, to provide sufficient lateral rigidity to prevent the tubing from kinking or collapsing while allowing it to flex laterally and longitudinally. The spiral rib may also be used to attach an endpiece to the tubing in a screw-like manner.
The tubing must either be disposable or sterilizable because medical practitioners typically replace it after about 72 to 168 hours of continuous use. Re-use of tubing through sterilization is increasing in response to concerns over damage to the environment by excessive medical waste.
Moreover, re-use of tubing maximizes economy. Sterilization is performed by exposing the tubing to heat ("heat-sterilization") or chemicals ("coldsterilization"). In heat-sterilization the tubing is typically exposed to steam at 270 degrees Fahrenheit for 20 minutes. Heat-sterilization is preferred because it is easier and more economical to perform than cold-sterilization.
Materials commonly used to form the flexible tubing include silicone rubber, high-density polyethylene, HYTREL.RTM., which is a thermoplastic copolymer produced by DuPont, Inc., and KRATON.RTM., which is a rubbery styrenic block copolymer or thermoplastic elastomer produced by Shell Chemical Company. Silicone rubber is durable and highly elastomeric. It may be reused many times over a lifetime of up to approximately fifteen years by removing it from the fittings, sterilizing it, and reattaching it to the fittings. (Hospitals, however, typically replace such tubing after three to five years because it becomes discolored and gives the appearance of uncleanliness.) In addition, the superior elastomeric and frictional properties of silicone rubber facilitate formation of a strong coupling. However, such tubing is relatively uneconomical because it is formed using a molding process. At present, a conduit between the ventilator and the patient wye made of silicone rubber tubing costs a hospital in the U.S. on the order of $150.
Polyethylene is considerably more economical than silicone rubber because it can be continuously extruded and then cut into the required lengths. Unlike silicone rubber, no expensive molding process is necessary. Nevertheless, it cannot be re-used in conjunction with either heat-sterilization or cold-sterilization. Not only will it melt or deform when subjected to the high temperatures of heat-sterilization, but polyethylene tubing that has been removed from a fitting for any reason cannot be re-attached to a fitting because polyethylene has little or no memory. Once stretched over a fitting, such tubing remains permanently stretched and cannot form a coupling with sufficient strength to pass the above-described attachment test. Therefore, polyethylene tubing must be discarded after a single use.
HYTREL.RTM. is more economical than silicone rubber, but it is not nearly as economical as polyethylene, both because the material itself is less economical than polyethylene and because it cannot be extruded like polyethylene. Although it can withstand high temperatures without melting or otherwise deforming, the tubing will harden over time when repeatedly heat-sterilized. Moreover, like polyethylene, it is not sufficiently elastomeric to be re-used by connecting it directly to a friction fitting. Silicone rubber endpieces are therefore screwed onto the ends of a length of HYTREL.RTM. tubing having a spiral rib and sealed with liquid silicone. The silicone rubber endpieces are sufficiently elastomeric and durable to be repeatedly reconnected to a fitting without degradation in the strength of the resulting coupling. (Silicone rubber has a tensile modulus of approximately 160 psi.)
KRATON.RTM. is nearly as economical as polyethylene because it can be extruded using thermoplastic processing methods. Like silicone rubber and HYTREL.RTM., it can withstand the temperatures of heat-sterilization. However, like polyethylene and HYTREL.RTM., it is not sufficiently elastomeric to be reused by connecting it directly to a friction fitting. (KRATON-D.RTM. has a tensile modulus between 400 and 1,000 psi.) Moreover, silicone rubber endpieces cannot be attached to the ends of a length of KRATON.RTM. tubing because KRATON.RTM. is too soft and pliable to form a strong spiral rib onto which an endpiece could be screwed and too resistant to adhesive bonding for liquid silicone or other adhesives and sealants to be used. (It is believed that a leaching agent in KRATON.RTM. prevents adhesion.) In attempts to secure KRATON.RTM. tubing directly to a fitting, plastic cable ties and rubber O-rings have been used as crude hose clamps. However, such clamping methods are inconvenient and unreliable. Moreover, succesive couplings using the same length of KRATON.RTM. tubing become weaker as repeated uses increasingly stretch the tubing. For these reasons, medical practitioners are reluctant to rely on such methods.
It would be desirable to provide a breathing circuit conduit that is not only relatively economical but can also be removed from a fitting, sterilized, and replaced in a fitting multiple times without suffering unacceptable degradation in the strength of the resulting coupling. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.